Hindsight that is pretty rare now.
At most of the riding schools I have known, it is normal for children to have a paid for hour lesson and spend the day at the stables doing stable management. Often older girls or adult helpers will teach them stable management skills, and many children enjoy being allowed to brush the ponies and so on. They also learn poo-picking/mucking out is part of taking care of a horse.
As they get older, and more useful to the riding school, it is more common for them to be offered extra riding (although most still pay for a lesson). This may range from acting as a "backmarker" on a hack (although usually only at 16+ if on the roads) to riding as lead file for less experienced lessons, to schooling younger/new horses (obviously only if their riding is up to the job!). Sometimes a completely free lesson or hack will be thrown in if they have helped with children's shows/parties/whatever, or the owner is just feeling generous.
7-8 is usually pretty much the minimum age that riding schools will allow child helpers, as before this children pretty much need constant adult supervision around horses. At 8, I would expect fairly inexperienced children to be constantly supervised around the horses by older teenagers/adults- by this I don't mean in the stable with them, but near enough that if the child shouts for help they will be heard.
Obviously, on a private yard, things will be a bit different, and there may not be any older helpers to supervise. I don't think this arrangement sounds brilliant, and your daughter may be better off going to a proper (BHS approved/PC centre) riding school, where you pay for lessons, and she does some supervised stable management. If cost is an issue, perhaps she could alternate weeks between this and the private yard?
Sorry this has turned into such a mega long post, but I do feel your daughter is being a bit exploited and may not be entirely safe in your current arrangement.